This invention relates to an injector for use in supplying fuel under pressure to a cylinder of an associated engine.
FIG. 1 illustrates a known fuel injector arrangement which comprises a fuel pump 1 including a plunger 2 which is reciprocable within a bore 3 under the action of a cam arrangement 4. A return spring 5 biases the plunger 2 out of the bore 3. The bore 3 communicates with a port of a spill valve 6, the other port of which communicates through a non-return valve 7 with a source of fuel.
The arrangement further comprises a two-stage lift injector 8 which includes a needle biased into engagement with a seating, the needle including surfaces oriented such that the application of fuel under pressure thereto tends to lift the needle from its seating. These surfaces are supplied with fuel through a supply line 9 from the bore 3 of the pump 1.
The needle carries a spring abutment 10 which engages a first spring 11, the other end of which engages a pressure backing member 12. The surface of the member 12 facing away from the spring 11 is exposed, through a passage 13, to the pressure at the said other port of the spill valve 6. A second spring 14 is located such that after a predetermined amount of movement of the needle away from its seating has occurred, further movement is opposed by both the first spring 11 and the second spring 14, thus a greater fuel pressure must be applied to the needle in order for such further movement to take place.
In use, in the position illustrated in FIG. 1, the bore 3 is charged with fuel, and the plunger is moving inwardly. The spill valve 6 is open, thus the inward movement of the plunger 2 results in fuel being displaced through the spill valve 6 and passage 13 to the member 12. Once the pressure of the fuel exceeds a predetermined pressure, the member 12 lifts against the action of the first spring 11, such movement permitting the fuel to escape to the spring chamber 15 and from there through a passage 16 to a low pressure reservoir. The fuel pressure necessary to move the member 12 is lower than that required to move the needle, thus whilst the spill valve 6 is open, injection does not commence. In order to commence injection, the spill valve 6 is closed. The fuel pressure applied to the member 12 falls, and the member 12 moves to its rest position under the action of the first spring 11. Continued inward movement of the plunger 2 pressurizes the fuel in the bore 3, thus the fuel pressure applied to the injector needle increases, and the increase is sufficient to result in movement of the needle from its seating against the action of the first spring 11 and subsequently against the action of both springs 11, 14. The movement of the needle causes a rod 10a carried by the spring abutment 10 to move into engagement with the member 12.
To terminate injection, the spill valve 6 is opened, thus permitting fuel at high pressure to be supplied to the passage 13. The pressure of fuel applied to the member 12 causes the member 12 and rod 10a to move which assists the spring 14 in moving the needle towards its seating against the action of the reduced pressure applied to the needle, and also permits fuel to flow to the passage 16. Continued inward movement of the plunger 2 therefore displaces fuel through the spill valve 6 to the low pressure reservoir. Once the plunger has completed its inward movement, the plunger 2 is withdrawn from the bore under the action of the spring 5, such movement drawing fuel through the non-return valve 7 and spill valve 6 to the bore 3.
In the arrangement illustrated in FIG. 1, the member is provided with an opening providing a restricted communication between the passage 13 and spring chamber 15, but this opening may be omitted.
It will be appreciated that the pressure which must be generated prior to injection to cause movement of the member 12 against the action of the first spring 11 is relatively high. It is an object of the invention to provide an arrangement in which this pressure is reduced.